In the automobile industry and also in pipe fabrication, hollow bodies, which in the axial direction have large changes of cross-sectional shape over only a short axial length, for example from a small cross-section to a large cross-section, are frequently needed. The shape transition between the axial portions, having different cross-sectional shapes, of these hollow bodies therefore essentially runs in a radial plane. An exhaust system of a motor vehicle, which has a correspondingly designed hollow body in the region of the silencer, can be regarded as a typical example. Other hollow bodies are, for example, supporting ends of a frame structure of a motor vehicle or fittings of pipe systems. All these hollow bodies are frequently produced by means of a plurality of interconnected, in particular welded, metal sheets, which have been formed into a corresponding hollow body. The strength of the hollow bodies produced in this manner is, however, in need of improvement due to the multiplicity of the welding seams. In addition, welding seams are susceptible to corrosion and increase the cost of production. Furthermore, a multiplicity of production methods is known in order to produce corresponding hollow bodies with steeply angled shape transition from a tubular body, for example. For example, it is also possible to produce shape transitions running radially by pressing circular sheet-metal discs or deep-drawn preform bodies. However, these shape transitions generally can only be produced at the ends of tubes and result in a pronounced thinning-out of the sheet thickness. The same is also true for internal high-pressure shaping, which requires additional high investment with respect to the tools. The rolling-in technique known, for example, for producing hollow bodies of arbitrary cross-section from a blank only permits shape transitions which run axially over a longer transition region to be produced. Shape transitions essentially running in a radial plane cannot be produced using the rolling-in technique. Finally, a shape transition running in a radial plane may be produced by hot forming. However, this method is also cost-intensive, in particular due to the necessary heating.
The published Japanese patent application JP 2005279706 A furthermore discloses a method for producing a hollow body with a shape transition in which an axial portion with a reduced cross-sectional area is drawn from a portion with a larger cross-sectional area using a forming punch inserted into the hollow body. On the one hand, the shape transition between the smaller and larger cross-sectional area is not affected in the method but rather the smaller cross-sectional region is drawn out. Furthermore, the sheet thickness reduction is relatively large in the known method.